Replace the single browse view with BrowseTabs (one tab per user),
mirroring the chat-room tabs. Browsing a new user (from search results
or a chat room's member list) opens or focuses their tab; Tab/Shift-Tab
switch between open users, 'w' closes the active tab, Esc hides the
popup while keeping the tabs. A tab bar shows the open users when more
than one is open, and each tab polls/loads independently. Re-browsing a
timed-out user retries. BrowseTabs is unit-tested.